The spindle pole duplication cycle of Saccharomyces cerevisiae provides a model system for genetic and molecular dissection of the functions required in centrosomal behavior and spindle pole regulation. Previous genetic characterization has identified three distinctive classes of functions that affect spindle pole body (SPB) duplication and coordination with other aspects of the cell cycle: One class -- represented by CDC4, CDC34 and CDC53 -- is required for a global function that mediates the transition from G1 to S phase. Cells mutant in this function form buds repeatedly while the duplicated SPBs fail to undergo separation for spindle formation. The second class, represented by the ESP1 and ESP2, is more directly implicated in regulation of the SPB cycle, as mutants undergo repeated cycles of SPB duplication without additional DNA replication or nuclear division. The third class, represented by the calcium-binding protein encoded by CDC31, is specifically required for SPB duplication, other early cell cycle functions occurring normally. These classes of functions are thus seen to play key roles in coordinating the duplication cycle with various other aspects of cell cycle progression. Detailed phenotypic analysis of alleles created by truncation and partial deletion of these genes in vitro will permit us to specify functional domains and to identify alleles defective in domain-specific functions. Suppressors and synthetic lethal mutations interacting with these alleles will then serve to identify novel gene products that help mediate these functions. Any new genes found will be cloned and analyzed for sequence and expression, as well as for immunological characterization of their products. The reagents generated in this work will provide a basis for probing protein:protein interactions in vitro and for dissecting the molecular mechanisms required for control of spindle pole behavior in the cell cycle.